Whenever we process data on a regular basis, especially if it takes a while to complete, we might run into a situation in which parallel requests want to trigger that process again, while the first is still running. Most of the time, this is not a good thing as it can lead to conflicts and/or data corruption. A good example from Drupal core in which this can happen is the cron. If we start it, the process can end up taking a good few seconds. Remember, it needs to pull together the hook_cron() implementations and run them all. So while that is happening, if we trigger another cron run, it will give us a nice message asking us to chill because the cron is already running. It does this with the help of the Lock API.
The Lock API is a low-level Drupal solution for ensuring that processes don't trample each other. Since in this chapter we are talking about things...
